The field of optoelectronic devices includes those which convert electrical energy into optical energy and those which convert optical energy into electrical energy. Such devices include photodetectors, phototransistors, solar cells, light emitting devices and lasers. Such devices typically include a pair of electrodes, referred to as a anode and cathode and at least one charge-carrying layer sandwiched between the anode and cathode. Depending on the function of the optoelectronic device the charge-carrying layer or layers may be comprised of a material or materials that are electroluminescent in response to an applied voltage across the electrodes or the layer or layers may form a heterojunction capable of generating a photovoltaic effect when exposed to optical radiation.
In particular, organic light emitting devices (OLEDs). are usually comprised of several layers in which one of the layers is comprised of an organic material that can be made to electroluminesce in response to an applied voltage, C. W. Tang et al., Appl. Phys. Lett. 51, 913 (1987). Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative to LCD-based full color flat-panel displays. S. R. Forest, P. E. Burrows and M. E. Thompson, Laser Focus World, February 1995. Some have sufficient transparency to be used in heads-up displays or in transparent windows and billboards. Significant commercial interest has been generated in a new type of display incorporating stacked organic light emitting devices that have the potential to provide high resolution, simple and inexpensive color displays and transparent color displays. V. Bulovic, G. Gu, P. E. Burrows, M. E. Thompson, and S. R. Forrest, Nature, 380, 29 (1996); U.S. Pat. No. 5,703,436, Forrest et al I. This transparent OLED (TOLED) had about 70% transparency when turned off, and it emitted light from both the top and bottom surfaces with a total device external quantum efficiency approaching 1% when the device was turned on. This TOLED used a transparent indium tin oxide (ITO) hole injecting layer as one electrode, the anode, and a Mg—Ag-ITO electron injecting layer as another electrode, the cathode. A transparency significantly greater than 70% would have been preferred, but the reflectance of the metal charge carrying layer prevented this.
A device was disclosed in which the ITO side of the Mg—Ag-ITO electrode was used as a hole injecting layer for a second, stacked TOLED. Additional layers could also be stacked, each layer being independently addressable and emitting a specified color. U.S. Pat. No. 5,707,745, Forest et al II, disclosed an integrated stacked, transparent OLED (SOLED) that allowed both intensity and color to be independently varied and controlled with an external power supply in a color tunable display device. Forrest et al II, thus, illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size of a SOLED. Furthermore, fabrication costs are comparatively less than prior art methods, making displays made from SOLEDs commercially attractive.
Such devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of injected electrons and holes. Specifically, OLEDs are comprised of at least two thin organic layers separating the anode and cathode of the device. The material of one of these layers is specifically chosen based on the material's ability to assist in injecting and transporting holes, a “hole transporting layer” (HTL), and the material of the other layer is specifically selected according to its ability to assist in injecting and transporting electrons, an “electron transporting layer” (ETL). In an optoelectronic device having at least one ETL and one HTL, the cathode is identified as the electrode on the ETL side of the device, and the anode is identified as the electrode on the HTL side of the device.
With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is more positive than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positively charged carriers) into the hole transporting layer, while the cathode injects electrons into the electron transporting layer. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. A Frenkel exciton is formed when an electron and a hole localize on the same molecule. One may visualize this short-lived state as having an electron that can drop, “relax,” from its conduction potential to a valence band, with relaxation occurring, under certain preferred conditions, by a photoemissive mechanism. Adopting this concept of the mechanism for operation of a typical thinlayer organic device, the electroluminscent layer comprises a luminescence zone receiving mobile charge carriers (electrons and holes) from the electrodes (cathode and anode).
The materials that function as the electron transporting layer or as the hole transporting layer of the OLED are frequently the same materials that are incorporated into the OLED to produce the electroluminescent emission. If the HTL or ETL function as the emissive layer of such devices, then the OLED is referred to as having a single heterostructure. Alternatively, an OLED, having a separate layer of electroluminescent material included between the HTL and ETL, is referred to as having a double heterostructure. Thus, a heterostructure for producing electroluminescence may be fabricated as a single heterostructure or as a double heterostructure.
One of the shortcomings in these OLEDs has been the transparency of the cathode. A high quantum efficiency is achieved using a metal layer with a low work function, such as magnesium-silver (Mg—Ag), calcium, or a compound electrode such as LiF—Al or LiAl, but the metal layer must be made thin enough to achieve a satisfactory transparency, because metal layers are also highly reflective and absorptive in the visible region of the spectrum. For example, a conventional TOLED uses a 75-100 Å Mg—Ag layer capped with a thicker layer of transparent ITO deposited on it. Although a device with about 70% transmission may be obtained, there is still significant reflection from the compound cathode. In addition, in SOLEDs in which at least one of the color producing layers is contained between the metallic cathodes of adjacent color producing OLEDs, microcavity effects are present which can give rise to color tuning problems. Z. Shen, P. E. Burrows, V. Bulovic, S. R. Forrest, and M. E. Thompson, Science 276, 2009 (1997). Such microcavity effects may also lead to an undesired angular dependence of the emitted light. Furthermore, thin Mg—Ag layers are sensitive to atmospheric degradation; therefore, they require special designs and processing steps to be undertaken so as to preserve their effectiveness in functioning as the cathode of an OLED.
In OLEDs where a still higher level of transparency is desired, a compound cathode comprising a non-metallic cathode and an organic interface layer can be used. Parthasarathy, P. E. Burrows, V. Khalin. V. G. Kozlov, and S. R. Forrest, Appl. Phys. Lett. 72, 2138 (1998) (“Parthasarathy I”). Due to the absence of a metallic cathode layer, the representative Alq3-based TOLEDs disclosed by Parthasarathy I emitted nearly identical light levels in the forward and back scattered directions. Optical transmission of at least about 85% was achieved using this non-metallic, compound cathode. However, the quantum efficiency of a device fabricated with such a cathode is typically reduced, in the range of about 0.1 to 0.3%, compared to OLEDS using the Mg—Ag-ITO cathode of Forrest et al I, wherein the device efficiency was about 1% but the transparency was only about 70%. Therefore, the non-metallic cathode improves transparency but degrades device efficiency. A cathode that is both highly transparent and efficient would be preferred.
It is known that a metal doped organic layer can be used in an OLED as an electron injecting layer at the interface between a metal cathode and an emitter layer to increase quantum efficiency of the OLED. A lithium doped layer of tris-(8-hydroxyquinoline) aluminum (Alq3) generates radical anions of Alq3, serving as intrinsic electron carriers, which result in a low barrier height for electron injecting and high electron conductivity of the lithium doped Alq3 layer. J. Kido and T. Matsumoto, Applied Physics Letters, v. 73, n.20, 2866 (1998). This improves quantum efficiency, but the device was not transparent.
A compound cathode comprising a layer of lithium-doped CuPc in contact with an emitter layer, such as α-napthylphenylbiphenyl (α-NPB), on one side and a layer of ITO, as a conductive layer, on the other side achieves an improved transparency and a slightly improved quantum efficiency, but lower efficiency than relatively non-transparent metal cathodes. L. S. Hung and C. W. Tang, Applied Physics Letters, v.74, n.21, 3209 (1999).
It would be desirable if compound cathodes could be fabricated from materials that were as transparent as the compound cathodes using ITO and CuPc or lithium-doped CuPc, but with the quantum efficiency of a metal cathode. This would combine high efficiency and high transparency in a single compound cathode that could be used in highly efficient and highly transparent optoelectronic devices.